Seal for an electrochemical cell

ABSTRACT

An electrochemical cell with a vent assembly that reliably prevents resealing of a vented seal member is provided. The vent assembly seals the end of a container that includes a first electrode, a second electrode, a separator located between the first and second electrodes and an electrolyte in contact with the electrodes and separator. The seal member includes a diaphragm with a hinge along one edge and a vent along another edge. The hinge and vent are located so that, relative to the longitudinal axis of the vent assembly, the hinge is closer to the vent than to the cell&#39;s terminal cover.

CROSS-REFERENCE TO A RELATED APPLICATION

This application is a continuation-in-part of prior application Ser. No.10/229,406, filed Aug. 28, 2002.

BACKGROUND OF THE INVENTION

This invention generally relates to ventable seals for pressurizedcontainers and, more particularly, to ventable seals for electrochemicalcells.

Electrochemical cells, such as cylindrical alkaline electrochemicalcells, employ two electrochemically active materials and an aqueouselectrolyte. The electrochemically active materials are typicallymanganese dioxide and zinc. These materials are conventionally housed ina cylindrical elongated container that is open on one end so that theelectrochemically active materials and electrolyte can be insertedtherein during the cell manufacturing process. A closure assembly thatincorporates a disc shaped polymeric seal member, a rigid inner coverand an elongated metallic current collector that projects through thecenter of the seal member, closes the open end of the container. Theseal member usually includes a hub, which surrounds the collector, and athin diaphragm integrally molded into the central region of the sealbody. The function of the diaphragm is to rupture and release gas fromwithin the cell when the internal pressure becomes too high. Thecollector provides a conductive path between the zinc and one of thecell's terminal covers which is located on an end of the cell.

Manufacturers of electrochemical batteries constantly strive to improvethe performance of their products in a wide variety of battery powereddevices. A key objective of most battery manufacturers is to improve theamount of time that a battery will power a device. One strategy toaccomplish this objective is to minimize the volume occupied by thecell's nonelectrochemically active components, such as the cell's ventassembly, thereby leaving more volume within the cell available for theelectrochemically active components. In recent years, vent assemblieshave been made more volume efficient by redesigning them to requirefewer components and yet operate more reliably than previously knownvent assemblies. As the volume occupied by the vent assembly hasdecreased, the problem of a vented seal “resealing” against anothercomponent of the vent assembly has become more apparent. This problemcan occur in primary batteries, also known as nonrechargeable batteries,if they are charged by other batteries. While most batteries are used ina conventional manner, a small percentage of batteries are exposed to anabusive condition such as when a primary battery is inadvertently placedin a device such that the battery's positive and negative terminals arereversed. This problem could occur, for example, in some commerciallyavailable flashlights that are powered by eight AA size alkalinebatteries. Typically, the batteries are inserted into two elongatedchambers containing four batteries each. According to the instructionsthat accompany the flashlight, the batteries must be inserted “inseries,” which means that the positive terminal of one battery contactsthe negative terminal of an adjoining battery. Unfortunately, someconsumers fail to comply with the instructions that are normallyincluded with the device and accidentally insert one of the cellsbackwards. If this occurs with the second or third battery in a cavitycontaining four batteries, then the cell inserted backwards will becharged by one or more of the properly inserted cells when the light'sswitch is moved to the “on” position. Charging the battery that has beeninserted backwards generates significant quantities of heat and gaswithin the battery. In some situations, the external temperature of thebattery could exceed 65° C. The increase in temperature increases thepressure within the cell. In addition to increasing the temperature ofthe battery, the chemical reactions that take place during the chargingof the cell generate gas that substantially increases pressure withinthe cell. The simultaneous production of gas and increase in temperaturecauses the polymeric seal, which is typically made of nylon, to becomesoft and lose some of its structural rigidity. The thin ventable portionof the seal may become elongated due to both the heating of the nylonand the increase in internal pressure. Consequently, when the softenedand distorted seal ruptures in response to the pressure buildup, aninitial quantity of gas may escape from within the cell but the tear inthe ruptured seal may be resealed when the softened ruptured sealcontacts the smooth inner surface of the terminal cover and resealsagainst the terminal cover. This problem is particularly acute with lowvolume vent assemblies wherein the distance between the seal member andother components, such as the cell's cover, is very small. If theruptured seal does reseal against the cover and the cell continues togenerate gas, the cell may eventually experience a crimp release whereinthe crimped connection between the seal and container is broken and thevent assembly is forcefully ejected from the container.

As disclosed in U.S. Pat. No. 6,270,919 B1, previous attempts to preventresealing of a ruptured seal body have included modifying a seal's innerdisc portion to include ribs. The ribs are designed to maintain theopening in a ruptured seal body thereby preventing resealing of the ventmechanism. However, while the inclusion of ribs in the seal's diaphragmis helpful in preventing resealing in most cells, some seals with theribs incorporated therein may be deformed when exposed to the heatgenerated during charging such that the ribs cannot maintain the openingin the seal after it has ruptured.

In a seal embodiment disclosed in U.S. Pat. No. 6,312,850 B1, verticalgrooves were placed in the surface of a compression member that forms apart of the seal assembly. The grooves are designed to prevent resealingof a vented seal's diaphragm. The grooves create channels that allow thegas to vent and thereby prevent resealing of the vented diaphragm. Whilethis embodiment does prevent resealing of the diaphragm against thesurface of the hub, the grooves in the compression member cannot preventresealing of the torn diaphragm against the interior surface of thecell's terminal cover.

U.S. Pat. No. 6,270,918 B1 discloses a seal assembly that utilizes aseal member and an inner cover with openings incorporated therein. Theseal member directly abuts the inner cover. An outer cover is secured tothe seal assembly and forms a contact terminal of the cell. If thecell's pressure increases beyond an acceptable limit, the seal memberruptures thereby allowing gas to escape through the openings in both theinner cover and contact terminal. While the described seal assembly doessafely vent an electrochemical cell, the seal assembly relies upon theuse of the inner cover. Unfortunately, the inner cover occupies spacewithin the cell that could be better used to store more of the cell'selectrochemically active materials.

Therefore, there exists a need for an inexpensive and simple tomanufacture polymeric vent assembly that occupies a minimum amount ofvolume within the cell and can reliably prevent resealing of a ventedseal against the electrochemical cell's terminal cover.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an electrochemical cell with a ventassembly that reliably prevents resealing of a vented seal member. Thevent assembly occupies a minimum amount of space within the cell and isinexpensive to produce.

In one embodiment, the electrochemical cell of the present inventionincludes an open ended container that includes a first electrode incontact with the container, a separator in contact with the firstelectrode, a second electrode in contact with the separator andelectrolyte in contact with the electrodes and separator. A ventassembly is secured to the open end of the container. The vent assemblyhas a longitudinal axis and includes a terminal cover, a seal member anda current collector. The terminal cover, which is electricallyconductive, has a recessed central region and a rolled back edge that isapproximately perpendicular to the central region. The seal member is arupturable disc shaped component made of a polymeric material. The sealmember includes an upstanding perimeter wall compressed between thecontainer and the cover's rolled back edge, and a terminal coverreception channel located adjacent the upstanding perimeter wall. Therolled back edge is disposed within the cover reception channel. Theseal member further includes a thin rupturable diaphragm region that hasa first edge and a second edge and is located between the seal member'supstanding perimeter wall and a centrally located hub. The hub definesan opening through the seal member. The seal member also includes ahinge which is located adjacent the first edge of the diaphragm regionand a vent which is located adjacent the second edge of the diaphragmregion. The hinge's elevation, relative to the longitudinal axis of thevent assembly, is closer to the elevation of the vent than to theelevation of the cover's central region. The vent assembly furtherincludes a vent deflection zone which is defined as the space below thecover, above the diaphragm region and between the diaphragm's first edgeand second edge. The zone is devoid of any structural element. Inresponse to an undesirable increase in the cell's internal pressure, theseal ruptures along at least a portion of the tine diaphragm's secondedge thereby creating a vent flap and an orifice through whichpressurized gas in the cell can escape. The flap rotates about the hingeand through the vent deflection zone without contacting any object thatwould cause the flap to block the release of the pressurized gas. Anelectrically conductive current collector contacts the hub that extendsthrough the opening in the seal member and contacts the cell's secondelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a nonvented seal member useful in a cell of thisinvention;

FIG. 2 is a cross-sectional view of a nonvented seal member useful in acell of this invention;

FIG. 3 is a longitudinal cross-sectional view of an electrochemical cellof this invention including the seal member shown in FIG. 1;

FIG. 4 is a top view of a vented seal member useful in a cell of thisinvention;

FIG. 5 is a cross-sectional view of a vented seal member useful in acell of this invention;

FIG. 6 is a longitudinal cross-sectional view of an electrochemical cellof this invention including the vented seal member shown in FIG. 5; and

FIG. 7 is an example of an electrochemical cell which is known in theprior art and includes a vent assembly with a support member.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and more particularly to FIG. 3, there isshown a cross-sectional view of an assembled electrochemical cell ofthis invention. Beginning with the exterior of the cell, the cellcomponents are the container 10, first electrode 12 positioned adjacentthe interior surface of container 10, separator 14 contacting theinterior surface 16 of first electrode 12, second electrode 18 disposedwithin the cavity defined by separator 14 and vent assembly 20 securedto container 10. Container 10 has an open end 22, a closed end 24 and asidewall 26 therebetween. The closed end 24, sidewall 26 and ventassembly 20 define a cavity in which the cell's electrodes are housed.

Shown in FIGS. 1 and 2 is an embodiment of seal member 28 which hasseveral sections that perform a variety of functions. As shown in FIG.3, the perimeter of seal member 28 is defined by upstanding perimeterwall 34 which forms an electrically insulating barrier between container10 which serves as the current collector for first electrode 12, andterminal cover 32 which serves as the external contact for secondelectrode 18. The interior surface of perimeter wall 34, also referredto herein as the upstanding wall, forms one side of a cover receptionchannel 76 in seal member 28 into which the rollback edge 36 of cover 32is inserted. Located adjacent the interior surface of rollback edge 36is buttress wall 38 which extends above the thin rupturable diaphragmregion 40. The function of buttress wall 38 is to oppose the forceexerted on diaphragm region 40 when the internal generation of gasincreases pressure within the cell. In a preferred embodiment, diaphragmregion 40 includes an elbow 42 which joins two annular sections ofdiaphragm 40 at an obtuse angle. By configuring the diaphragm so that itis bowed toward the interior of the cell, the pressure exerted on thediaphragm by the buildup of gas within the cell results in the diaphragmbeing compressed. When the cell's internal pressure exceeds apredetermined value, the diaphragm inverts and tears at vent line 44which is a groove in the interior surface of a seal member 28. Vent line44 is aligned with the outer circumferential surface of compressionbushing 46, so that compression bushing 46 facilitates tearing of thediaphragm at vent line 44.

When diaphragm 40 responds to an undesirable increase in internalpressure by inverting and tearing at vent line 44, diaphragm 40 forms avent flap 48, see FIGS. 5 and 6, with a distal end 50 and a proximateend 52. Vent flap 48 is specifically designed to rotate toward terminalcover 32 so that orifice 78 is created in seal member 28. Pressurizedgas within the cell escapes to the environment beyond the cell bypassing through the orifice in the seal member and then through opening54 in terminal cover 32. Vent flap 48 is made to rotate around hingearea 56, which is located at the proximate end 52 of vent flap 48. In apreferred embodiment, hinge area 56 forms an arc that is concentric withthe central opening 30 in seal member 28.

Hinge 56 is the arc shaped line in the seal member about which the ventflap rotates when the seal is ruptured. The hinge coincides with atleast a portion of first edge 64 of diaphragm 40. Preferably, the hingeforms an arc or a circle that is concentric with the vent assembly'slongitudinal axis. Abutting and defining another edge of diaphragm 40 issecond edge 66. First edge 64 and second edge 66 are concentric with oneanother and the vent assembly's longitudinal axis.

Preferably, second edge 66 comprises a grooved section and a nongroovedsection. The grooved section defines the location of vent 44. Thenongrooved section forms a nonventable interface between a portion ofdiaphragm 40 and hub 70. The nonventable interface forms anchor 82. Thefunction of anchor 82 is to secure vent flap 48 to both diaphragm region40 and hub 70, thereby ensuring that vent flap 48 does not contactterminal cover 32 and form a 360° seal against the interior surface ofthe terminal cover when the cell's internal pressure causes seal member28 to rupture at vent line 44. The arc formed by vent line 44 should beat least 180°, more preferably approximately 220° and most preferablynot more than 250°. The portion of diaphragm 40 that abuts hub 70 anddoes not include vent line 44 forms an anchor 82. The arc formed byanchor 82 should be at least 30°, more preferably approximately 100° andmost preferably less than 180°. Upon activation of the vent, orifice 78is formed by tearing the diaphragm at vent line 44. Orifice 78 mayextend from one end of vent line 44 to the other end of vent line 44provided orifice 78 does not extend into the nongrooved section ofsecond edge 66. Alternatively, orifice 78 may extend around a portion ofvent line 44 as shown in FIG. 4. The size of orifice 78 is not criticalprovided it is large enough for the gas within the cell to rapidlyescape when the cell vents. The arc of anchor 82 and the arc of ventline 44 form a complete circle.

Relative to the elevation of the vent assembly's longitudinal axis 58,the elevations of hinge 56, terminal cover 32 and vent line 44 in sealmember 28 are critical to preventing the resealing of vent flap 48against the interior surface of cover 32. If the distal end 50 of ventflap 48 is allowed to contact cover 32 and the cell continues togenerate gas after the diaphragm has ruptured, as would occur when acell has been inserted backwards in a multicell device and issubsequently charged by the other cells, then the flap is forcefullypressed against cover 32 thereby blocking further release of gas throughthe seal member and cover. If the internal pressure exceeds thecontainer's ability to remain crimped over the perimeter of the sealmember, then the vent assembly may be rapidly expelled from thecontainer. Some of the cell's other components, such as electrolyte andportions of the anode, may also be forcefully thrown from the container.

To prevent resealing of a vented seal member, the elevation of the hingemust be closer to the elevation of the vent than to the elevation of thecover's central portion. The elevations of the hinge, vent and cover aredetermined relative to the longitudinal axis 58 of vent assembly 20. Asshown in FIG. 3, the longitudinal axis 58 of the vent assembly isconcentric with the longitudinal axis of container 10. For use in thisapplication, the highest elevation within the vent assembly is thesurface of the terminal cover's central portion. Any point along thevent assembly's axis that is between the terminal cover and the end ofcurrent collector 30 that contacts second electrode 18 is considered tobe lower than the highest elevation. The elevation of any feature in thevent assembly is determined by connecting a straight line from thefeature in question to the vent assembly's longitudinal axis providedthe axis and straight line meet at a right angle. When comparing theelevation of any two features in the vent assembly, the feature with itsstraight line closest to the surface of terminal cover 32 is consideredto be higher in elevation than the other feature.

As shown in FIG. 3, the elevations of cover 32, hinge 56 and vent 44,relative to the longitudinal axis of the vent assembly, correspond topoints A, B and C, respectively. In the vent assembly of the presentinvention, the elevation of hinge 56 (point B) must be closer to theelevation of vent 44 (point C) than to the elevation of the junction ofcollector 30 and the cover's central portion (point A). Preferably, theelevation of hinge 56 is greater than the elevation of vent 44. Thephysical relationships of the cover, hinge and vent are necessary toensure that the distal end 50 of vent flap 48 does not contact andreseal against cover 32 when excessive pressure within the cell causesseal member 28 to rupture. During the rupturing of seal member 28,diaphragm 40 rapidly inverts and tears at vent line 44. The portion ofthe diaphragm located between hinge 56 and vent 44 forms the vent flap48 which is forced to rotate through the vent deflection zone 60, towardcover 32, by the pressurized gas as it escapes from the cell. As usedherein, the vent deflection zone 60 is defined as the space below theterminal cover, above diaphragm region 40, and between the diaphragm'sfirst edge 64 and second edge 66. In FIG. 3, the vent deflection zone isidentified with parallel diagonal lines. To ensure proper operation ofthe vent assembly, the vent deflection zone 60 should be devoid of anystructural element that would impede movement of the vent flap and causethe flap to block the release of pressurized gas from the cell. Forexample, the existence of a support member 62, shown in FIG. 7, wouldinhibit proper operation of the vent flap. Support member 62 contactsthe seal member's upstanding wall and the hub thereby compressing theperimeter wall against the container and the hub against the currentcollector. Consequently, in a preferred embodiment, the vent assembly ofthis invention includes a seal member, a current collector and aterminal cover that also functions as one of the cell's terminals and noother structural element in the vent deflection zone.

Inwardly disposed from second edge 66 are hub base 68, hub 70 andcollector 30. Hub base 68 is thicker than diaphragm 40. Hub 70 abuts hubbase 68 at a right angle. Hub 70 defines central opening 30 in sealmember 28.

Seal member 28 is typically made as a single component by injectionmolding a polymeric material into a cavity and then allowing the part tosolidify. Materials from which the seal member may be made include:nylon 6, 6; polypropylene; and impact modified polystyrene as describedin U.S. Pat. No. 6,306,537.

An optional component of the vent assembly shown in FIGS. 3 and 6 iscompression bushing 46. The exterior circumferential surface 72 of thecompression bushing is made to align with vent 44. In vent assembliesthat do not include a compression bushing, the vent is made to alignwith the outside diameter of hub 70.

Terminal cover 32 is a generally cup-shaped component made ofnickel-plated steel. The perimeter 36 of cover 32 is formed by rollingback the edge of the cover upon itself to create a double thickness ofmaterial that is approximately perpendicular to the central portion 74of cover 32. Between the covered central portion and its perimeter, thecover is contoured through a series of bends. The cover's central region74 is recessed relative to the cover's rolled back edge 36. When cover32 is inserted into the seal member's cover reception cavity 76 and thevent assembly is crimped into the open end of container 10, the coveracts like a spring that compresses the seal member's upstanding wall 34against the interior surface of container 10. By contouring the cover topossess a spring like characteristic, the vent assembly does not need toinclude a separate component to force the seal member's perimeter wallagainst the container.

First electrode 12 is a mixture of manganese dioxide, graphite and anaqueous solution containing potassium hydroxide. The electrode is formedby disposing a quantity of the mixture into the open ended container andthen using a ram to mold the mixture into a solid tubular shape thatdefines a cavity which is concentric with the sidewall of the container.Alternatively, the cathode may be formed by preforming a plurality ofrings from the mixture comprising manganese dioxide and then insertingthe rings into the container to form the tubularly shaped firstelectrode.

Second electrode 18 is a homogenous mixture of an aqueous alkalineelectrolyte, zinc powder, and a gelling agent such as crosslinkedpolyacrylic acid. The aqueous alkaline electrolyte comprises an alkalinemetal hydroxide such as potassium hydroxide, sodium hydroxide, ormixtures thereof. Potassium hydroxide is preferred. The gelling agentsuitable for use in a cell of this invention can be a crosslinkedpolyacrylic acid, such as Carbopol 940®, which is available from Noveon,Inc., Cleveland, Ohio, USA. Carboxymethyylcellulose, polyacrylamide andsodium polyacrylate are examples of other gelling agents that aresuitable for use in an alkaline electrolyte solution. The zinc powdermay be pure zinc or an alloy comprising an appropriate amount of one ormore of the metals selected from the group consisting of indium, lead,bismuth, lithium, calcium and aluminum. A suitable anode mixturecontains 67 weight percent zinc powder, 0.50 weight percent gellingagent and 32.5 weight percent alkaline electrolyte having 40 weightpercent potassium hydroxide. The quantity of zinc can range from 63percent by weight to 70 percent by weight of the anode. Other componentssuch as gassing inhibitors, organic or inorganic anticorrosive agents,binders or surfactants may be optionally added to the ingredients listedabove. Examples of gassing inhibitors or anticorrosive agents caninclude indium salts (such as indium hydroxide), perfluoroalkyl ammoniumsalts, alkali metal sulfides, etc. Examples of surfactants can includepolyethylene oxide, polyethylene alkylethers, perfluoroalkyl compounds,and the like.

The second electrode may be manufactured by combining the ingredientsdescribed above into a ribbon blender or drum mixer and then working themixture into a wet slurry.

Electrolyte suitable for use in a cell of this invention is athirty-seven percent by weight aqueous solution of potassium hydroxide.The electrolyte may be incorporated into the cell by disposing aquantity of the fluid electrolyte into the cavity defined by the firstelectrode. The electrolyte may also be introduced into the cell byallowing the gelling medium to absorb an aqueous solution of potassiumhydroxide during the process used to manufacture the second electrode.The method used to incorporate electrolyte into the cell is not criticalprovided the electrolyte is in contact with the first electrode 12,second electrode 18 and separator 14.

During the cell assembly process, a quantity of an aqueous potassiumhydroxide solution is incorporated into the cell. The solution serves asthe cell's electrolyte. The electrolyte may be incorporated into thecell by allowing the gelling agent in the second electrode to absorb theelectrolyte during the electrode manufacturing process which occursprior to inserting the second electrode into the coiled separator. Inaddition, a quantity of electrolyte may be dispensed into the partiallycompleted cell any time after the first electrode has been inserted intothe container and prior to closing the cell.

Separator 14 may be made from nonwoven fibers. One of the separator'sfunctions is to provide a barrier at the interface of the first andsecond electrodes. The barrier must be electrically insulating andionically permeable.

The above description is considered that of the preferred embodimentsonly. Modifications of the invention will occur to those skilled in theart and to those who make or use the invention. Therefore, it isunderstood that the embodiments shown in the drawings and describedabove are merely for illustrative purposes and are not intended to limitthe scope of the invention, which is defined by the following claims asinterpreted according to the principles of patent law, including theDoctrine of Equivalents.

1. An electrochemical cell, comprising: (a) an open ended container,said container comprises a first electrode in contact with saidcontainer, a separator in contact with said first electrode, a secondelectrode in contact with said separator and electrolyte in contact withsaid electrodes and said separator; and (b) a vent assembly, having alongitudinal axis, is concentrically aligned with and secured to theopen end of said container, said vent assembly comprises: i) anelectrically conductive cup shaped terminal cover having a recessedcentral region and a rolled back edge that is approximatelyperpendicular to the central region; ii) a disc shaped rupturable sealmember made of a polymeric material, said seal member comprises anupstanding perimeter wall compressed between said container and saidcover's rolled back edge, a terminal cover reception channel locatedadjacent the upstanding perimeter wall, said rolled back edge disposedwithin said cover reception channel, and a thin rupturable diaphragmregion having a first edge and a second edge, said diaphragm locatedbetween the seal member's upstanding wall and a centrally located hubwhich defines an opening through said seal member, said seal membercomprises a hinge located adjacent a first edge of the thin diaphragmregion and a vent located adjacent a second edge of the thin diaphragmregion, said hinge's elevation, relative to the longitudinal axis of thevent assembly, is closer to the elevation of the vent than to theelevation of the cover's central region; iii) a vent deflection zone,said zone defined as the space below the cover, above the diaphragmregion and between the diaphragm's first edge and second edge, said zonedevoid of any structural element, wherein in response to an undesirableincrease in the cell's internal pressure, said seal ruptures along atleast a portion of said thin diaphragm region's second edge therebycreating a vent flap and an orifice through which pressurized gas in thecell can escape, said flap rotates about the hinge and through the ventdeflection zone without contacting any object that would cause the flapto block the release of said pressurized gas; and iv) an electricallyconductive current collector contacts said hub, extends through theopening in said seal member and contacts said second electrode.
 2. Theelectrochemical cell of claim 1 wherein said second edge of said thindiaphragm region comprises a grooved section and a nongrooved section,said grooved section defining the location of said vent and saidnongrooved section defining an anchor which secures said diaphragmregion to said hub.
 3. The electrochemical cell of claim 2, wherein saidgroove is arc shaped and concentrically aligned with the central openingin said seal member.
 4. The electrochemical cell of claim 2, whereinsaid nongrooved section forms at least a 30° arc.
 5. The electrochemicalcell of claim 2, wherein said nongrooved section forms approximately a100° arc.
 6. The electrochemical cell of claim 2, wherein saidnongrooved section forms less than a 180° arc.
 7. The electrochemicalcell of claim 2 wherein said grooved section forms an arc greater than180°.
 8. The electrochemical cell of claim 2 wherein said groovedsection forms an arc greater than 220°.
 9. The electrochemical cell ofclaim 2 wherein said grooved section forms an arc greater than 250°. 10.The electrochemical cell of claim 1 wherein said hinge's elevation isbetween the elevation of the vent and the elevation of the cover'scentral region.
 11. The electrochemical cell of claim 1 wherein saidhinge is arc shaped and concentrically aligned with the central openingin said seal member.
 12. The electrochemical cell of claim 1 whereinsaid seal member comprises a buttress wall abutting said diaphragmregion and said cover when said cell is pressurized, said buttress wallpositioned to resist the compressive force exerted on said diaphragm byan increase in the cell's internal pressure.
 13. The electrochemicalcell of claim 1 wherein said polymeric material is selected from thegroup consisting of nylon, polypropylene and impact modifiedpolystyrenes.
 14. The electrochemical cell of claim 1 wherein saidstructural element contacts said seal member's upstanding wall and saidhub.
 15. The electrochemical cell of claim 1 wherein said structuralelement compresses said hub against said current collector.
 16. Theelectrochemical cell of claim 15 wherein said structural elementcompresses said perimeter wall against said container.